1. Field of the Invention
This invention relates to the field of octane improvement of a hydrocarbon stream and more specifically to the octane improvement of naphtha produced by Fischer-Tropsch synthesis.
2. Background of the Invention
Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. However, the transportation over large distances may require refrigerated, pressurized vessels. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing, and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that, in addition to their value, they can be transported more easily in a pipeline or in large vessels than natural gas, since they do not require the energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen and/or steam to form synthesis gas, which is a combination of carbon monoxide and hydrogen. In the second transformation, which is known as Fischer-Tropsch synthesis, carbon monoxide is reacted with hydrogen to form organic molecules containing mainly carbon and hydrogen. Those organic molecules containing carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen, which are known as oxygenates, can also be formed during the Fischer-Tropsch synthesis. Hydrocarbons comprising carbons having no ring formation are known as aliphatic hydrocarbons and are particularly desirable as the basis of synthetic diesel fuel.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus has a range of molecular weights. Therefore, the Fischer-Tropsch products produced by conversion of synthesis gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield naphtha, diesel, and jet fuel, as well as heavier middle distillates. Hydrocarbon waxes may be subjected to an additional hydroprocessing step for conversion to a liquid and/or a gaseous hydrocarbon. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reaction zone that may include one or more reactors.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors, either placed in series, in parallel or both. Common reactors include packed bed (also termed fixed bed) reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors comprising catalytic particles sometimes called “slurry reactors,” “ebullating bed reactors,” “slurry bed reactors” or “slurry bubble column reactors,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Some of the principal advantages of gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) for the exothermic Fischer-Tropsch synthesis are the very high heat transfer rates, and the ability to remove and add catalyst online. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55), incorporated herein by reference in its entirety, give a history of the development of various Fischer-Tropsch reactors.
The naphtha produced typically is comprised mainly of C5 through C11 linear alkanes. Such material has low octane value and typically requires processing to upgrade for use in gasoline formulations. Therefore, the naphtha is typically used as a feedstock for a steam cracker. In the steam cracker, the light ends of the naphtha are broken down into olefins, such as ethylene, propylene and butenes. Drawbacks include low yields for heavier fractions. In addition, drawbacks include the production of coke.
Consequently, there is a need for improving the octane number of a Fischer-Tropsch naphtha. A further need exists for an improved process for increasing the octane number of a Fischer-Tropsch naphtha.